实体框架 - 将检索到的Combobox值转换为字符串

Question

使用Entity Framework，我有一个Combobox，显示从数据库中检索的数据列表。

using System;  //I removed the other using statements here to preserve space

namespace ExTea_BackEnd
{
    public partial class frmAddBreakdown : Form
    {
        ExTeaEntities Breakdowns;
        Breakdown_Type BreakdownTypes;

        public frmAddBreakdown()
        {
            InitializeComponent();
        }

        private void cmbBreakdownType_SelectedIndexChanged(object sender, EventArgs e)
        {
            Breakdown_Type breakdownType = (Breakdown_Type)cmbBreakdownType.SelectedItem;
            string selectedBreakdownTypeId = breakdownType.BrkdwnId;

            IQueryable<Breakdown_Type> breakdownTypeQuery = from t in Breakdowns.Breakdown_Types
                                                            where t.BrkdwnId == selectedBreakdownTypeId
                                                            select t;
            List<Breakdown_Type> selectedBreakdownId = breakdownTypeQuery.ToList();

            if (selectedBreakdownId != null && selectedBreakdownId.Count > 0)
            {
                BreakdownTypes = selectedBreakdownId.First();
                txtBreakdownId.Text = BreakdownTypes.BrkdwnId.ToString();
            }
            else
            {
                BreakdownTypes = null;
            }
        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            try
            {
                Breakdown newBreakdown = new Breakdown();

                Breakdown_Type breakdownType = (Breakdown_Type)cmbBreakdownType.SelectedItem;
                newBreakdown.BrkdwnType = breakdownType.ToString();   //this is where the error occurs

                newBreakdown.MachineId = txtMachineId.Text.Trim();
                newBreakdown.MachineType = txtMachineType.Text.Trim();
                newBreakdown.ReportedDate = dtpDate.Value;
                newBreakdown.JobStatus = "I";

                Breakdowns.AddToBreakdowns(newBreakdown);
                int rowsAffected = Breakdowns.SaveChanges();
                if (rowsAffected > 0)
                {
                    MessageBox.Show(rowsAffected + " records added!", "Save", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error occured! " + ex.Message);
            }
        }
    }
}

我正在尝试将表单中的数据保存回数据库，尝试转换从Combobox中选择的值时会发生错误。即使我将它转换为正确的类型，它也不会保存所选的值！但是这个，

我对这里的错误一无所知？有人可以告诉我如何纠正这个问题吗？

谢谢。

Answer 1

newBreakdown.BrkdwnType = breakdownType.ToString();

这里你只是调用你的对象的.ToString()方法，所以它返回你可以在数据库表记录中查看的类型名称，用于获取BrkDwnType属性值，你应该将语句改为

newBreakdown.BrkdwnType = breakdownType.BrkdwnType; 

Answer 2

它应该只是：

            newBreakdown.BrkdwnType = cmbBreakdownType.SelectedItem.ToString(); 

如果你只想要字符串值，

不需要转换selectedItem。从设计的角度来看，您应该规范化您的数据库，并使用一个ID来获取引用具有BreakDown类型的另一个表的breakDownType。